The present disclosure relates to a semiconductor structure and a method of forming the same. More particularly, the present disclosure relates to a metal oxide semiconductor field effect transistor (MOSFET) having a high performance replacement gate structure with reduced gate-contact parasitic capacitance, and methods of manufacturing the same.
Since its first implementation in 45 nm technology node, a high-k metal gate stack including a high-k gate dielectric (i.e., a gate dielectric having a dielectric constant of greater than 3.9, typically greater than 7.0) and a metal gate has become a common process platform in advanced complementary metal oxide semiconductor (CMOS) devices.
One of the process schemes for fabricating a high-k/metal gate MOSFET is a replacement gate method. In a replacement gate process, a MOSFET can be fabricated using a disposable gate structure. In such a process, the disposable gate structure is formed first and thereafter the disposable gate structure is replaced by a gate stack including a high-k gate dielectric and a gate electrode. Since the gate stack including the high-k gate dielectric and the gate electrode is formed after high temperature processing steps such as a source/drain activation anneal, the replacement gate process has the advantage of minimal damage on the high-k gate dielectric and the gate electrode. Moreover, a wide range of metals can be selected for the gate electrode.
Because the high-k gate dielectric and the gate electrode “replace” the disposable gate structure by filling a U-shaped recessed region, i.e., gate cavity, formed after removal of the disposable gate structure, the high-k gate dielectric follows the contour of the recessed region. The presence of a high-k dielectric material on the sidewalls of the U-shaped recess results in a significant parasitic capacitance between the gate electrode and the source and drain regions of a replacement gate MOSFET, thereby adversely impacting the performance of the replacement gate MOSFET.
Further, replacement gate MOSFETs typically employ a work function metal portion in each gate electrode such that the work function metal portion contacts the high-k gate dielectric. The work function metals, however, have a greater resistivity than other conductive materials, such as aluminum or tungsten, that are deposited on the work function metals and fill a predominant portion of the electrode-shaped recessed region. While a horizontal portion of the work function metal portion contacting a top surface of a high-k gate dielectric is required in order to adjust threshold voltage of the replacement gate MOSFET, vertical portions of the work function metal portion located on sidewalls of a gate electrode and laterally surrounding the other conductive material merely increase the resistance of the gate electrode.
Therefore, there remains a need to provide a MOSFET structure having a high performance replacement gate structure with reduced gate-contact parasitic capacitance.